Since the discovery of antitumor activity of cisplatin, i.e., cis-(NH.sub.3).sub.2 PtCl.sub.2 by B. Rosenberg (Nature, 205,698 (1965)), comprehensive studies including its clinical tests have been performed leading to FDA approval of cisplatin as a chemotherapeutic antitumor agent in 1979. Currently, cisplatin is one of the most widely used antitumor agents and, in particular, is effective for testicular, ovarian, bladder, lung, bone marrow, and larynx cancers, but because of its high toxicity (LD.sub.50 =13 mg/kg, M. J. Cleare, Biochimie, 60, 835(1978)), its use is limited. On the other hand, carboplatin, i.e., cis-(NH.sub.3).sub.2 Pt(CBDCA)(CBDCA=1,1-cyclobutanedicarboxylate) which was approved by FDA in 1989, has much lower toxicity (LD.sub.50 =180 mg/kg, M. J. Cleare, Biochimie, 60 (1978)) compared with cisplatin, but its antitumor activity is lower and more expensive than cisplatin. Therefore, a great deal of researches for searching new antitumor agents having a higher antitumor activity and lower toxicity than those of cisplatin or caroplatin is actively underway.
With a view to develop novel platinum antitumor agents having a higher antitumor effect and lower toxicity than those of cisplatin, the present inventors modified the molecular structure of anion and nuetral amine ligand of cisplatin to synthesize new platinum complexes and examined the physiological activity thereof. As a result, the present inventors has found that the platinum complexes bound by a malonic acid derivative containing a double bond at .alpha.-position exhibited an excellent antitumor activity and filed an application for the malonic acid derivatives platinum complexes containing sulfur atom as Korean patent application No. 91-11401 filed Jul. 5, 1991 (the corresponding U.S. application has been granted as the U.S. Pat. No. 5,142,075) wherein A.dbd.A'.dbd.NH.sub.3, CH.sub.3 NH.sub.2, C.sub.2 H.sub.5 NH.sub.2, iso-C.sub.3 H.sub.7 NH.sub.2, cyclo-C.sub.3 H.sub.5 NH.sub.2, AA'.dbd.NH.sub.2 CH.sub.2 CH.sub.2 NH.sub.2, 1,2-(NH.sub.2).sub.2 -cyclo-C.sub.6 H.sub.10, CH(OH)(CH.sub.2 NH.sub.2).sub.2 ; RR.dbd.--SCHCHS--, SCH.sub.2 CH.sub.2 S-, R.dbd.SCH.sub.3). The present inventors have continued the studies and found that the platinum complexes represented by the formula I containing malonic acids without sulfur of the following general formulas II-A and II-B as anion group have superior antitumor activity to that of carboplatin and cisplatin and much lower toxicity than that of cisplatin. Moreover, the compounds of formula I of the present invention exhibit an improved water-solubility. Thus, it is possible to solve the problems as troubled in the use of cisplatin due to the low water-solubility. ##STR2##